The Ordovician of Argentina

WAISFELD, B.

Congreso; Annual Meeting IGCP 635, IGCP 735, Subcommission on Ordovician Stratigraphy; 2021

Univdersity of Lille

Widespread early Paleozoic exposures, locally over 6.000 m thick, extending from Northeast Patagonia to Venezuela, represent the remnants of the early history of the active Proto-Pacific margin of Gondwana. Major geological units include autochthonous cratonic basins developed on and around the Gondwana basement (e.g. Cordillera Oriental and Sierras Subandinas that extend into Bolivia and Perú) and early Palaeozoic subduction-related volcanic arcs and associated volcano-sedimentary basins fringing Gondwana (e.g. Famatina?Puna arc, further extended to northern and central Andes). As well, crustal fragments accreted to the Andean margin of Gondwana through the Paleozoic are also involved (e.g. Cuyania terrane). In the context of South America, Ordovician successions and faunas from Argentina are comparatively well-known. At the Northwest of Argentina, a large-scale retroarc foreland basin system developed during the Ordovician. From West to East this system includes several depozones represented by the Puna (volcanic arc and foredeep), Cordillera Oriental (peripheral bulge), and Sierras Subandinas (back bulge). A wealth of stratigraphical and paleontological information comes from the Cordillera Oriental. Widespread Furongian (Stage 10) ?Dapingian successions are interpreted as shallow marine deposits punctuated by valley incisions and deltaic progradation. They provide interesting insights on diversity patterns and ecosystem structure during the interval. Middle Ordovician records and Hirnantian glacigenic deposits crop out as isolated remnants. The Sierra de Famatina, the southern prolongation of the Puna magmatic arc, is regarded as a major volcanic chain, largely at sea level, located on continental crust. Preserved stratigraphy encompass Furongian (Stage 10) to Tremadocian siliciclastic rocks and Floian-Dapingian volcanic and volcanoclastic deposits in discontinuous and largely disconnected outcrops, increasingly deformed towards the North of the basin. This over 3000 m thick succession has been interpreted as a back arc foreland basin fill. Environmental conditions in this arc-related setting triggered differential diversity trends among disparate fossil groups. Finally, the Cuyania microplate (extended Precordillera terrane) has been interpreted as a drifted-rifted crustal block from the southeastern Laurentia that accreted to western Gondwana by Middle-Late Ordovician time. Its most outstanding feature is the c. 2500 m thick succession of Cambrian (basal Series 2) to Middle Ordovician passive-margin, shallow water platform carbonates. This succession is overlain by siliciclastic deposits and olistostromes representing deeper slope and basinal environments, finally capped by Hirnantian glacial deposits. Ordovician stratigraphy preserved in Precordillera is almost complete and facies and faunas broadly mirror its drifting, collisional, and post-collisional history. In sum, there is a lot of disparity in the preserved stratigraphic record of each basin, as well the stratigraphical and paleontological knowledge is uneven among them because of multiple reasons (tectonic deformation, depositional gaps, dense forest cover, lack of fossils, etc.). Global chronostratigraphical stages of the Ordovician System and their boundaries are currently recognized, but with different degrees of resolution depending on the region and interval considered. In general terms, the biota, patterns of diversification and paleoecological signals are fairly idiosyncratic to each basin probably due to contrasting geodynamic contexts and depositional settings.